In the prior art it is well known that ovenizing or heating a quartz crystal oscillator can stabilize the oscillator frequency. The prior art for directly heated ovenized quartz crystal oscillators includes the following: Tinta, Matistic, and Lagasse “The Direct Temperature Control of Quartz Crystals in Evacuated Enclosures” IEEE 24th Annual Symposium on Frequency Control, 1970; U.S. Pat. No. 3,715,563 to Bloch; U.S. Pat. No. 4,748,367 to Tanuma et al.; U.S. Pat. No. 4,091,303 to Takataka et al.; U.S. Pat. No. 4,985,687 to Long; U.S. Pat. No. 3,431,392 to Garland et al.; and U.S. Pat. No. 3,818,254 to Persson.
Example products on the market are manufactured and sold by Valpey Fisher, Statek, Micro Crystal Switzerland, Vectron and others.
These oscillators suffer from a number of deficiencies. First, strains generated due to the thermal mismatch of the quartz and the metal electrodes, especially near the heater, may be transmitted to an active region, which can shift the fundamental resonance frequency and negatively affect the oscillator stability.
Also temperature uniformity is heavily dependent on heater geometry. Slight spatial variations in the thickness of the heater metal and/or heater width can lead to spatial variations in the dissipated power density in the heater, which can result in spatial temperature non-uniformities that negatively affect oscillator stability.
In certain designs with better temperature uniformity, the heater electrode may directly oppose a signal electrode, which can excite undesired resonance in unintended regions of the quartz crystal and negatively affect frequency stability.
Existing ovenized oscillators consume >200 mW of power at −20° C., have warm-up times greater than 30 seconds, and have a frequency stability ranging from 100 s to 1/10 s of ppb, with smaller oscillators having poorer performance.
Other more traditional ovenized oscillator designs and products exist, but they consume even higher power than directly heated quartz oscillators. For example, see OCXOs for Portable Equipment (Is a directly heated oscillator right for you?) by Greg Arthur Feb. 1, 2008, www2.electronicproducts.com/OCXOs for portable equipment-article-facnvalpey-feb2008-html.aspx.
The prior art for micromachined quartz crystal oscillators and resonators includes: U.S. Pat. No. 7,559,130 to Kubena et al.; U.S. Pat. No. 7,237,315 to Kubena et al.; U.S. Pat. No. 7,555,824 to Chang et al.; US Published Patent Application No. 20090189294 by Chang et al.; US Published Patent Application No. 20080258829 by Kubena et al.; US Published Patent Application No. 20070216490 by Kubena et al.; and US Published Patent Application No. 20070205839 by Kubena et al.
What is needed is a directly heated oversized quartz crystal oscillator that is small, and which has a low power consumption, a fast warm-up period, and improved frequency stability. Also needed is a method of making such a quartz crystal oscillator. The embodiments of the present disclosure answer these and other needs.